A known evaporative emission control system for a vehicle that is powered by a naturally aspirated internal combustion engine comprises a vapor collection canister having an inlet port in communication with the headspace of the fuel tank, an outlet port that is communicated through a canister purge valve with the induction air intake system at a location in the air intake system that is downstream of the throttle, and an atmospheric, or vent, port that is communicated to atmosphere. Volatile fuel vapors generated in the tank headspace are collected in the collection canister, and when the vehicle is operating under conditions that are conducive to canister purging, the collected vapors are conveyed by the purge control valve to the air intake system to entrain with the induction flow into the combustion chamber space of the engine. It is a typical practice to place the purge control valve under the control of an engine management computer which determines when and in what amount purging can occur. The purge flow is induced by the subatmospheric pressure that is created in the intake system so that in effect the collected vapors are sucked by the engine from the canister.
In the case of an engine which has some type of supercharging device, such as a blower or turbocharger, that is selectively operable to increase the power output of the engine over that which the engine is capable of producing when operating naturally aspirated, such an evaporative emission control system will be incapable of purging the canister during times of supercharged operation because the pressure in the intake system at the location therein to which the canister is communicated is now superatmospheric rather than subatmospheric. Thus a supercharged engine is confronted with the problem that such an associated evaporative emission control system will be incapable of purging the canister during periods of supercharged operation.
One known solution to this problem is to provide either a pneumatically operated switch valve which is operated by intake manifold pressure, or an electrically operated switch valve, to route the purge flow either upstream or downstream of the supercharger depending upon the activity of the supercharger.
The present invention relates to a new and unique solution to this problem which does not require the inclusion of a switched valve as in the foregoing solution. Indeed, the present invention can be implemented by the simple and relatively inexpensive expedient of providing a conduit from the atmospheric, or vent, port of the canister to a location in the air intake system that is upstream of the supercharger and throttle plate, preferably between the supercharger and an air filter that filters incoming airflow. During engine operation wherein the pressure in the air intake system below the throttle is subatmospheric, the purge system operates exactly the same as in the case of a strictly naturally aspirated engine. During engine operation wherein the pressure in the air intake system below the throttle is superatmospheric, the purge system operates to cause the purge flow to be conveyed out the atmospheric port, through the newly added conduit, to the intake system where the purge flow entrains with the filtered air flow entering the supercharger. During supercharged operation, the canister purge flow is induced by the combined effect of superatmospheric pressure downstream of the throttle acting on the canister purge outlet port through the canister purge valve and of subatmospheric pressure between the air filter and the supercharger acting on the canister atmospheric, or vent, port, and therefore, the purge control valve will also be effective to control the canister purge flow during supercharged engine operation. The term "supercharger" is used herein in a comprehensive sense to include any powered device that is capable of creating superatmospheric pressures in the intake system.
The foregoing features, advantages, and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which are accompanied by a drawing. The drawing discloses a presently preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.